1. Field of Art
This invention relates to surface acoustic wave signal processing, and more particularly to apparatus for linear surface acoustic wave signal processing.
2. Description of the Prior Art
In commonly owned U.S. Pat. No. 4,024,480 to Reeder and Grudkowski, a surface acoustic wave (SAW) signal processor utilizes frequency mixing diodes having direct amplitude and phase control to provide nonlinear frequency mixing, the amplitude of which is controllable by DC bias applied to the nonlinear diodes. One application described in said patent employs pre-programmed DC bias for the different taps to provide amplitude (as well as phase) correlation of the various chips (corresponding to related taps) of a phase shift keyed (PSK) signal train. Another application disclosed in said patent is multiplexing of DC signals representative of parameters such as time and temperature, by applying the DC signals to the respective nonlinear mixer circuits through the DC bias paths, to provide a sequence of RF signal chips, each chip having amplitude and/or phase indicative of the parameter represented by the DC signals. An additional application, discrete Fourier transform of a discrete coded signal, is shown in commonly owned U.S. Pat. Nos. 4,106,514 Reeder and Gilden and 4,114,116 to Reeder.
However, in all of these disclosed applications (and others) the operation is nonlinear with respect to the DC signals. As is common in the art, the diode bias is chosen for maximum mixer efficiency (mixer efficiency being the gain characteristic utilized in frequency mixing, SAW tap circuitry). In this operating regime, the RF voltage output varies inversely with the square of the DC bias current, such that the RF power output varies inversely with the fourth power of the DC input signal. In use of such devices for correlation, the pre-programmed biases may, of course, be suitably adjusted so as to accommodate the inverse square mixer efficiency. And, when using such circuitry for analysis or multiplexing of parameter-related DC signals, it is possible to accommodate nonlinear outputs by means of conversion circuitry or the like. However, these functions become much simpler, and more easily adapted to limited space, controlled-weight environments and the like, when linearity (between RF output and DC bias) is inherently provided. Further, the cost and complexity of design and maintenance of such equipment is naturally reduced significantly if inherent linear operation is utilized.